问题补充：
We all know that dinosaurs lived on the Earth more than years ago.
A. sixty million
B. millions of
C. sixty million of
D. million of
答案：
(答案→)A. sixty million
B. millions of
C. sixty million of
D. million of
解析：句意：我们都知道恐龙在地球上生存在6千万年前。表示确切数字，million不用复数；表示概数，million必须用复数，并且和介词of连用，所以选A。
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